Recently, “an alumina/spinel castable” comprising a combination of a raw alumina material and a raw spinel material and “an alumina/magnesia castable” obtained by adding magnesia to an alumina castable were developed as “refractories for the lining of furnaces used at high temperatures” such as molten-steel ladles including VOD ladles, RH vacuum degassers, tundishes for hot operating, covers of smelting furnaces, ash melting furnaces, and cement preheaters, and are being used in such applications. Of these, the “alumina/magnesia castable” is attracting attention as a material especially for ladle lining because of reduced slag infiltration, and is coming to be applied to the bottom and side wall.
Incidentally, “unshaped materials comprising a raw alumina material, raw magnesia material, alumina cement, and ultrafine silica powder” are described as “alumina/magnesia castables” in JP-A-63-218586, JP-A-2-208260, JP-A-5-185202, and JP-A-7-25669.
Compared to the alumina castable and alumina/spinel castable heretofore in use, the unshaped materials described in those patent documents are superior in corrosion resistance and unsusceptibility to slag infiltration. However, with recent changes in circumstances surrounding the iron and steel industry, the treatment of molten steels in ladles is becoming severer so as to improve steel quality, and the improvement attainable with the unshaped materials described above has reached a limit. There also are limitations on measures for the steady “need of the iron and steel industry for a refractory cost reduction by life prolongation”.
Under those circumstances, the present inventors proposed “a spinel/magnesia castable obtained by compounding spinel (MgO.Al2O3) and magnesia as the main raw materials with a raw spinel material as an aggregate part and with small amounts of a raw alumina material and a raw magnesia material as fine powder parts” prior to the present application (see Taikabutsu, Vol. 54, No. 1 (published by the Technical Association of Refractory, Japan in January, 2002) pp. 23–24). This spinel/magnesia castable not only has high unsusceptibility to infiltration and hot-stress relaxation characteristics but also can improve corrosion resistance while inhibiting infiltration, like the alumina/spinel castable, due to the incorporation of spinel also as an aggregate part.
The invention relates to an improvement of the “spinel/magnesia castable”. An object of the invention is to provide an unshaped refractory composition which, in particular, not only has high heat resistance but also has far higher corrosion resistance and unsusceptibility to slag infiltration than the alumina, alumina/spinel, or alumina/magnesia castable heretofore in use.